Molecular Clouds in in the LMC at High
- Slides: 7
Molecular Clouds in in the LMC at High Resolution: The Importance of Short ALMA Baselines T. Wong 1, 2, 4, J. B. Whiteoak 1, M. Hunt 2, J. Ott 1, Y. -N. Chin 3 1 CSIRO Australia Telescope National Facility 2 School of Physics, University of New South Wales, Australia 3 Tamkang University, Taiwan 4 Contact: Tony. Wong@csiro. au
• Australia Telescope Compact Array Six 22 m antennas near Narrabri, NSW, five moveable on rail tracks. • National Facility open to proposers worldwide. • Operates in 5 frequency bands from 1 -25 GHz. • 3 mm (85 -105+ GHz) upgrade in progress for 5 antennas (due late June). • Wide-bandwidth (2 GHz x 4 IF) correlator under development (mid-2006). Longitude 150° E, Latitude 30° S
The ATCA Millimetre Upgrade • 3 elements currently have dual linear polarization 3 mm receivers, 5 by July. • 2 observing bands: 84. 9 -87. 3 and 88. 591. 3 GHz. Full coverage of 85 -105 GHz expected by July, extension to 115 GHz planned. • A~0. 35, Tsys~300 K (above atmosphere). (above) 3 mm lownoise amplifiers based on In. P MMIC technology (left) Both 3 mm and 12 mm systems are housed in a single dewar.
Molecular Clouds in the LMC A unique nearby, low-metallicity star formation environment. Contours: CO at 2. 6’ resolution from NANTEN (Mizuno et al. ) N 113 HII region SEST spectra (Chin et al. 1997)
Mapping of N 113 in 4 transitions HCO+ (1 -0) C 2 H (N=1 -0) HCN (1 -0) HNC (1 -0) • We observed N 113 in HCO+, HCN, HNC, C 2 H, and 87 GHz continuum. • Observations conducted in 2003 July & August in two E-W configurations of 3 antennas (baselines 30 -135 m). • RMS noise ~30 m. Jy in a 2 km s-1 channel. Integrated intensity images for 4 lines. Contour levels: 0. 5 Jy bm-1 km s-1 for top panels and 0. 2 for bottom panels. • Reference pointing on Si. O maser R Dor, phase calibration using PKS B 0537 -441 (25° away).
Comparison in u-v plane • Assuming a SEST gain of 25 Jy/K, the total HCO+ and HCN fluxes are 80 and 60 Jy km s-1 (Chin et al. 1997) respectively. • Thus only ~15% of flux is detected on the shortest (30 m) ATCA baseline. • The relative sizes of the emission regions differ: HCO+ is more extended than HCN, which in turn is more extended than HNC. • Possible explanations: HCO+ associated with an extended PDR, HNC/HCN enhanced in dense cores.
Implications for ALMA DRSP • Emission from abundant species like HCO+ and HCN is heavily resolved, even on a 30 m baseline. • Information on abundances & cloud structure will require observations from an array of smaller dishes (ACA). • Lack of small-scale emission implies high-resolution observations will require much greater sensitivity. • Note that ATCA’s field of view at 90 GHz (36”, see figure) is similar to ALMA’s at 230 GHz (~10 pc at LMC distance) — mosaicking clearly needed.
